A serum protein specific to the fetus was first demonstrated by Pederson, Nature (London) 154, 575 (1944) in calf serum and by Bergstrand et al. Scand. J. Clin. Lab. Invest 8 174 (1956) in humans. This feto-specific protein was designated alpha.sub.1 -fetoprotein (AFP) due to its electrophoretic mobility. Alpha.sub.1 -fetoprotein in sera of hepatocellular cancer and malignant embryonal teratoma patients was found to be immunologically and electrophoretically identical with alpha.sub.1 -fetoprotein observed in fetal serum by Tatarinov, Vopr. Med. Khim. 11 20-24 (1965) and Abelev et al., Int. J. Cancer 2 551-558 (1967).
Human alpha.sub.1 -fetoprotein has been isolated from fetal serum by chemical procedures by Pederson, Clin. Chimica Acta 38 163-170 (1971), Ruoslahit et al., Int. J. Cancer 7 218-225 (1971), Silver et al., Proc. Nat. Acad. Sci., U.S.A. 70 526-530 (1973), Forrester et al., Clin. Chimica Acta 64 317-323 (1975) and Twomey et al., Clin. Chem. 22 1306-1309 (1976). In addition immunochemical and chemical methods have been employed to isolate alpha.sub.1 -fetoprotein from hepatoma serum by Nishi, Cancer Res. 30 2507-2513 (1970), Lehmann et al., Clin. Chimica Acta 33 197-206 (1971) and Alpert et al., J. Biol. Chem 247 3492-3497 (1972).
The methods referred to above result in an alpha.sub.1 -fetoprotein (AFP) which is not pure in that it contains albumin and other proteins. Because of the presence of these contaminants, such relatively impure alpha.sub.1 -fetoprotein preparations are not satisfactory as the radiolabeled antigen for a sensitive radioimmunoassay. Further, such preparations are not satisfactory for the production of monospecific high titer antisera for a sensitive radioimmunoassay (RIA). Because samples to be tested for AFP in a radioimmunoassay often contain AFP in low concentration, it is essential that the RIA procedure be highly sensitive. By highly sensitive is meant that the RIA must be able to accurately detect AFP at a level of about 20 ng/ml of sample. Such sensitive RIA procedures are required for screening for birth defects in pregant women, which has heretofore not been feasible.
Non-specific antisera or impure labeled AFP cannot be utilized in such sensitive RIA procedures since they would not yield an accurate measure of the AFP content of the sample. In addition, the methods of alpha.sub.1 -fetoprotein reported in the literature are not satisfactory for isolating AFP from large volumes of source solutions and/or source solutions which contain low concentrations of AFP. Further, the methods disclosed in the literature for isolating AFP do not appear to be satisfactory for automation techniques.
There is thus a need for a method of isolating alpha.sub.1 -fetoprotein in as pure a state as possible in terms of albumin and other protein contaminants which would interfere with sensitive radioimmunoassays. Further, there is a need for isolating AFP which can be efficiently applied to large volumes of source solutions and/or source solutions which contain AFP in low concentration. Finally, there is a need for a method of isolating AFP which is amenable to being carried out, wholly or partially, by automated apparatus and techniques. These needs are all satisfied by the method of the subject invention. Further, the methodology of the present invention affords a means whereby AFP can be obtained from monkey hepatoma serum in sufficient purity so as to be for practical purposes, the same as that obtained from human cord serum and sufficiently pure to be used in RIA procedures on human samples. Monkey AFP has heretofore not been recognized as being usable in radioimmunoassays on human material.